1. Field of the Invention
The present invention relates to a method for removing impurities from porous materials and, more particularly, to a method for removing impurities from a porous material at the nanometer level by utilizing a supercritical fluid.
2. Description of the Related Art
Currently, keeping component surfaces clean, and ensuring that new materials are properly purified, are important aspects to improve product yields and reliability, and have become very serious problems for high technology industries. Prior art cleaning processes include the use of acidic or alkaline solvents, strong oxidizing solvents, and organic solvents. These cleaning processes have been used for a long time and are effective, but they also are a considerable source of problems. For example, the cleaning processes invariably require huge amounts of water and chemical reagents, which often leads to product and environmental contamination, while further requiring a following drying process. However, when the components have deep channels, high aspect ratios or are made of porous media, the prior art cleaning processes cannot efficiently clean these detailed structures on the above-mentioned components, nor are they able to enter into nanometer scale holes due to the large surface tension of the prior art cleaning solvents. Therefore, there are residual contaminants and moisture after the cleaning processes. Additionally, the components must undergo a drying process after the cleaning process; however, during the drying process, the surface tension of the prior art cleaning solvent may cause pattern collapse on the components and destroy the original structure of the component, causing deteriorated characteristics. Moreover, the drying process requires a significant amount of time, which affects the following processes. Therefore, the prior art solvent cleaning processes are unsuitable for materials with complicated structures or for porous materials.
A supercritical fluid has physical properties that are between the gas phase and the liquid phase. A supercritical fluid has low viscosity (requiring less transportation power than a liquid), like a gas, a high diffusion coefficient (a diffusion coefficient 10 to 100 times higher than a liquid, which has a lower mass transfer resistance than liquid so that its mass transfer is faster than a liquid), a low surface tension (it easily penetrates into porous materials), and a high density like a liquid (thus supplying more supercritical fluid than a gas; this also increases a residence time for a fluid in a reactor, which is better adapted for a continuous operation). Not only the physical properties, but also the chemical properties of a supercritical fluid are different than the gas phase or the liquid phase. For example, the gas CO2 has no extraction abilities, but supercritical CO2 is lipophilic, and is able to dissolve organic material that changes with different temperature and pressure adjustments. Spent supercritical fluid can revert back to the gas phase by reducing its pressure, thereby separating from other solid and liquid phase materials, and thus can be easily recycled, which is one of the advantages of a supercritical fluid. CO2 is one of most popular and important of various fluids, because its critical conditions are moderate, its critical point is easily achieved, its critical temperature is 31.2° C., which is close to room temperature, and its critical pressure is about 72.8 atm. Furthermore, CO2 is not toxic, not combustible, is stable, and can be obtained from petroleum fuel combustion waste products, and so is inexpensive.
There are three patents that use supercritical fluid to remove impurities in porous low dielectric films (low k). U.S. Pat. No. 6,306,754 uses supercritical fluid to clean impurities and photoresist residuum left in a porous low dielectric film after etching; the supercritical fluid is ethane or CO2, and is mixed with alcohol, ketone or a combination of the two as a modifier, and has an operating pressure of 70–200 atm and an operating temperature of 35–100° C. U.S. Pat. No. 6,669,785 uses a supercritical fluid to remove oxide, etching residuum or photoresist residuum and contaminants left in a porous low dielectric material; the supercritical fluid is CO2 that includes a first fluid and a second fluid, with an operating density of 0.150 g/cc–1.1 g/cc, an operating temperature of 0–80° C., and a co-solvent that can be amine, morpholine, aniline, dibutylamine or C1–C4 alcohol in 0.1–40% w/w, and with a surfactant at a ratio of 0.1–5% w/w. U.S. No. 2004018452 uses a supercritical fluid mixed with a passivation agent to clean etching residuum left in a porous low dielectric surface material to avoid material deterioration; the supercritical fluid is CO2, and the passivation agent is an acid or fluoride. After analyzing the above-mentioned patents, it is obvious that the supercritical fluid used for cleaning porous low dielectric films is primarily CO2, but which requires mixing with different modifiers having different properties and proportions. The removed contaminants are primarily photoresist, photoresist residuum, etching residuum, which are organic materials. There is no patent that provides a technology to remove moisture, or that uses the same modifier to clean moisture, organic contaminants and surface modifications. For a porous low dielectric film, the etching process may cause the film material to deteriorate; moisture remaining in the apparatus can cause the dielectric constant to increase. The inability to simultaneously remove organic contaminants and moisture limits both the cleaning procedures and the development of porous materials in an integrated fabrication process.
Therefore, it is desirable to provide a method for removing impurities and moisture from porous materials to mitigate and/or obviate the aforementioned problems. Further, it has not been known using supercritical fluid to clean carbon nanotubes.